The present invention relates to improved membrane electrode assemblies and fuel cells with long lifetime, comprising two electrochemically active electrodes separated by a polymer electrolyte membrane.
In polymer electrolyte membrane (PEM) fuel cells, the proton-conducting membranes used nowadays are almost exclusively sulfonic acid-modified polymers. Predominantly perfluorinated polymers are employed. A prominent example thereof is Nafion™ from DuPont de Nemours, Wilmington, USA. For proton conduction, a relatively high water content in the membrane is required, which is typically 4-20 molecules of water per sulfonic acid group. The water content needed, but also the stability of the polymer in conjunction with acidic water and the hydrogen and oxygen reaction gases, limits the operating temperature of the PEM fuel cell stacks to 80-100° C. Higher operating temperatures cannot be achieved without loss of performance of the fuel cell. At temperatures above the dew point of water for a given pressure level, the membrane dries out completely, and the fuel cell no longer supplies any electrical energy since the resistance of the membrane rises to such high values that there is no longer any significant current flow.
For system-related reasons, however, higher operating temperatures than 100° C. in the fuel cell are desirable. The activity of the noble-metal-based catalysts present in the membrane electrode assembly (MEA) is much better at high operating temperatures.
More particularly, in the case of use of what are called reformates from hydrocarbons, distinct amounts of carbon monoxide are present in the reformer gas and typically have to be removed by a costly and inconvenient gas processing or gas cleaning operation. At high operating temperatures, the tolerance of the catalysts to the CO impurities rises.
In addition, heat arises in the operation of fuel cells. However, cooling of these systems to below 80° C. can be very costly and inconvenient. According to the power released, the cooling apparatus can be made much simpler. This means that, in fuel cell systems which are operated at temperatures above 100° C., the waste heat can be utilized much better and hence the fuel cell system efficiency can be enhanced.
In order to attain these temperatures, membranes with novel conductivity mechanisms are generally used. One approach to doing this is the use of membranes which exhibit ionic conductivity without the use of water. A first promising development in this direction is described in the publication WO 96/13872. A further high-temperature fuel cell is disclosed in the publication JP-A-2001-196082.
In addition, WO 02/088219 discloses a second generation of high-temperature fuel cell based on polyazoles, which are produced by condensation polymerization in polyphosphoric acid (PPA) and partial hydrolysis of the same reaction mixture. These proton-conducting polymer membranes exhibit improved properties compared to the membranes known from WO 96/13872. Nevertheless, these membranes too can still be improved for long-term operation in a high-temperature fuel cell. Especially in the case of sustained use temperatures of 160-180° C. and frequent startup and shutdown of the fuel cell, degradation or aging of the membrane cannot be ruled out. Under some circumstances, this degradation can lead to an irreversible failure of the membrane electrode assembly.